Transimpedance amplifiers (TIA) are commonly used for providing a voltage output signal proportional to a current signal and are normally implemented by providing a feedback resistor across the input and output nodes of an operational amplifier. TIAs are often known as current to voltage converters. One use of a TIA is to convert an input current signal into an output voltage signal.
FIG. 1 shows a transimpedance amplifier 100 implemented with MOSFETs. The TIA comprises an n-channel MOSFET (NMOS) 110 and a p-channel MOSFET (PMOS) 120 arranged such that the gate terminals 112, 122 of the MOSFETs are tied to an input vin, and the drain terminals 114, 124 of the MOSFETs are tied to an output vout. The source terminal 126 of the PMOS is connected to a current source 130 and a discharge capacitor 140. A feedback resistor 150 is connected across the TIA 100. With this arrangement, the gain Gm of the amplifier is controlled by the current IS provided to the TIA by the current source 130 where Gm∝√{square root over (IS)}.
In RF receiver modules, TIAs generally function as low-noise pre-amplifiers, which largely determine the overall performance of the module. In the past, because of the wide bandwidth and the high gain necessary for sensitive data links, TIAs have been implemented with bipolar and gallium arsenide (GaAs) metal-semiconductor field effect transistors (MESFET). These implementations result in high speed devices, but they can be costly and the production processes lack high-yield manufacturability. Recently, metal-oxide-semiconductor (MOS) technology has become popular for the design of TIAs because of its low cost and high-yield manufacturability. However, using a single MOS gain stage fails to provide enough gain for multi-gigabit operation, because MOS transistors have a lower transconductance than bipolar transistors. Hence, successful high-speed MOS implementations have relied on multiple gain stages.
At present, the current supplied to a TIA is fixed at a maximum value that ensures the highest gain Gm(max) from the TIA. However, this gain is not always required and so during normal operation of the TIA, this high current is mostly taken up by the load. This unnecessarily increases the power consumption of the TIA when Gm(max) is not required.
In view of the above, there remains a need to provide a TIA which is driven by a current source that is adaptive to the requirements of the TIA circuit such that a high current source is only provided to a component of the circuit (when Gm(max) is required) while supplying the remaining components of the circuit with lower current sources so as to minimise the overall power consumption.